(Note: The current UI doesn't reflect this yet, the upcoming updates will add this)

Although Epic items are guaranteed to to give you an Epic power cell, you can get those also by scrapping lower rarity items, they don't guarantee you a higher rarity however. There is a trick however.

You can increase the chances of getting a higher rarity power cell from scrapping lower rarity items by evolving them up before scrapping.

Scenario:

Say you need that Epic power cell. Evolve a Rare item to Evo 2 (level 11). Scrapping it guarantees you an 1 Epic power cell or better. It can be a Legendary power cell as well instead of the Epic.

The following basic rule applies:

Common Evo 3 Level 21+ when scrapped gives guaranteed 1 or more Uncommon power cell or better.

CAUTION: As always, be careful to not scrap the items you want to keep. Scrapping destroys your items and you won't get them back!​

NOTE: This rule does not apply for epic items scrapping to gain legendary power cells. Epic items past Evo 2 guarantee you bonus epic power cells and increase the chance for the bonus epic cells to be legendary cells instead. There is no guarantee for epic items to give you legendary power cells.​